Blood pressure monitoring system and method of having an extended optical range

ABSTRACT

A system and method for monitoring blood pressure of a wearer has an inflatable arm cuff that is selectably inflatable to differing air pressures that incorporates a fabric having both a light transmission property and a light reflection property when the fabric is illuminated with light having wavelength(s) in the range from about 400 to about 2200 nanometers. A radiation source and a detector are attached to the fabric in relative positions such that the reception of incident radiation by the detector is directly affected by a change in the amount of light transmitted through the fabric relative to the amount of light reflected by the fabric as the fabric stretches in response to motion in the wearer&#39;s body due to changes in the flow of blood through an artery disposed beneath the fabric occurring in consonance with variations in the air pressure within the inflatable cuff.

CROSS-REFERENCE TO RELATED APPLICATIONS

Subject matter disclosed herein is disclosed in the following co-pendingapplications:

System for Monitoring Motion of a Member, U.S. Application No.60/502,760; (LP-5345USPRV), filed Sep. 12, 2003 in the name of Chia Kuoand George W. Coulston.

Blood Pressure Monitoring System and Method, U.S. Application No.60/502,751; (LP-5347USPRV), filed Sep. 12, 2003 in the names of GeorgeW. Coulston and Thomas A. Micka.

Reflective System for Monitoring Motion of a Member, U.S. ApplicationNo. 60/502,750; (LP-5346US PRV), filed Sep. 12, 2003 in the name ofGeorge W. Coulston;

Blood Pressure Monitoring System and Method Having Extended OpticalRange, U.S. Application No. 60/526,187; (LP-5622USPRV), filed Dec. 2,2003 in the names of George W. Coulston and Thomas A. Micka.

Extended Optical Range Reflective System for Monitoring Motion of aMember, U.S. Application No. 60/526,429; (LP-5621 USPRV), filed Dec. 2,2003 in the name of George W. Coulston.

Extended Optical Range System for Monitoring Motion of a Member, U.S.Application No. 60/526,188; (LP-5620USPRV), filed Dec. 2, 2003 in thename of Chia Kuo and George W. Coulston.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system and a method for monitoring bloodpressure using a fabric having predetermined optical properties thatrespond to the motion(s) generated by geometric changes in a body due tothe conduction of blood through an artery located beneath the fabric.

2. Description of the Prior Art

The “pulse” of the heart is associated with pressure pulses known toexist in the arteries carrying blood throughout the body. The periodicpumping of the heart produces these pressure pulses, which in turn flexthe artery walls in rhythm with the pumping of the heart. The maximum,or peak, pressure exerted against the arterial wall occurs during thesystole phase of the beat and is termed “systolic pressure”. The lowest,or baseline, pressure (known as the “diastolic pressure”) occurs duringthe diastole phase of the beat.

As the heart beats the pressure in the arteries fluctuates (higherduring the systole phase and lower during the diastole phase of eachbeat), and is best described by the values for the systolic anddiastolic pressures. Typical practice is to express blood pressure as aratio of the maximum and minimum values.

The generally known method to determine these two blood pressureextremes is the auscultatory method. In this method a pressure cuff isapplied to a person's upper arm. This cuff includes a bladder capable ofholding air at a predetermined known pressure. The cuff bladder isinflated to a pressure above the highest expected pressure to bemeasured, i.e., above the systolic pressure. When inflated at thishighest pressure, the cuff prevents the flow of blood in the brachialartery of the arm underlying the pressure cuff. The bladder is equippedwith a valve, which allows the pressure to be reduced in a controlledway.

As air is released from the bladder, blood flow in the brachial arteryis re-established. The inflow of blood through the artery is accompaniedby pulsing sounds known as the Korotkoff sounds. These sounds aredetected using a stethoscope at a point on the brachial artery justbelow the pressure cuff. The falling pressure in the cuff bladder isobserved while air is released.

The Korotkoff sounds are divided in five phases based on loudness andcertain qualitative features. The five phases of the Korotkoff soundsare also identified with certain pressure regimes, as normal arterialblood flow is being re-established. The first phase of the Korotkoffsounds (Phase 1) is heard at about 120 millimeters of mercury (mm Hg)characterized by a sharp “thud”; this is the systolic (maximum) bloodpressure. Phase 2 is identified with a pressure of about 110 mm Hg andis heard as a swishing or blowing sound. Phase 3 is identified with apressure of about 100 mm Hg and is described as a thud that is softerthan that of Phase 1. At a pressure of about 90 mm Hg the firstdiastolic pressure is detected, called the Phase 4 Korotkoff sound, andheard as a softer blowing sound which disappears. Phase 5 is identifiedwith about 80 mm Hg, and is called the second diastolic pressure. Thislast phase is silent, meaning that a laminar blood flow has been againestablished. Phase 5 may be absent in some human subjects. For thisreason, the first diastolic pressure of Phase 4 is recorded as thelowest pressure in the artery.

An automated auscultatory apparatus relies on detecting sound levels andcomplex processing of these sounds into electronic signals, which arecorrelated with the phases of the Korotkoff sounds. Representative of anautomated arrangement that uses an auscultatory method is themeasurement system disclosed in U.S. Pat. No. 6,511,435 (Bluth et al.)

The re-establishment of blood flow in an occluded artery is alsoaccompanied by a relatively significant flexure of the arterial wall.The flexure diminishes as the artery widens with the decrease in cuffpressure. In an alternative form of blood pressure measuring apparatus,known as an oscillatory measurement system, the mechanical vibrationsaccompanying arterial wall flexure are transformed into sound as theyenter the inflated bladder of the cuff. This sound is detectable using amicrophone located in the bladder. U.S. Pat. No. 6,458,085 (Wu et al.)discloses an oscillatory blood pressure measurement arrangement. Anoscillatory blood pressure measurement arrangement is also disclosed inthe above-mentioned U.S. Pat. No. 6,511,435 (Bluth et al.).

One drawback of oscillatory measuring apparatus is the reliance upon acuff bladder modified to contain a microphone and associated connectionsto an external signal processor. Using a microphone to detect changes insound pressure level and recognizing a pattern from the waveform sogenerated is difficult.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for monitoringblood pressure by detecting motion due to geometric changes in thewearer's body.

The present invention is usable with an inflatable arm cuff that isselectably inflatable to differing air pressures and incorporates afabric which has both a light transmission property and a lightreflection property when the fabric is illuminated with light havingwavelength(s) in the range of from about 400 nanometers to about 2200nanometers, and particularly in the ranges from about 400 to about 800nanometers and from about 700 to about 2200 nanometers. The amount oflight transmitted through the fabric relative to the amount of lightreflected by the fabric is able to change when the fabric stretches inresponse to motion in the wearer's body due to changes in the flow ofblood through an artery disposed beneath the fabric.

A radiation source that emits radiation with wavelength(s) in the rangefrom about 400 nanometers to about 2200 nanometers, and particularly inthe ranges from about 400 to about 800 nanometers and from about 700 toabout 2200 nanometers, and a radiation detector are attached to thefabric in relative positions such that the reception of incidentradiation by the detector is directly affected by a change in the amountof light transmitted through the fabric relative to the amount of lightreflected by the fabric as the fabric stretches in response to motion inthe body of a wearer due to changes in the flow of blood through anartery disposed beneath the patch occurring in consonance withvariations in the air pressure within the inflatable cuff. A pressurerecorder responsive to the signal output from the detector records thepressure of the cuff when the output of the detector is at a minimumvalue and the pressure in the cuff following the minimum when the signalfrom the detector again lies within a predetermined range of a baselinesignal value. In one embodiment, the fabric forms a patch that isdisposed on or over the inflatable arm cuff.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription, taken in connection with the accompanying drawings, whichform a part of this application, and in which:

FIG. 1 is a schematic diagram of a system useful for the monitoringblood pressure of a subject by sensing geometric changes in the body ofthe subject due to the motion of blood passing through blood vesselsunderlying a patch of monitoring fabric;

FIGS. 2A and 2B are diagrammatic views illustrating possible lighttransmission and reflection response of the fabric used in themonitoring system of the present invention during normal systole anddiastole phases of heart action;

FIGS. 2C and 2D are respective diagrammatic views illustrating possiblelight transmission and reflection response of the fabric when a bloodvessel underlying a patch formed of such fabric is totally occluded andwhen blood flow is re-established;

FIG. 2E is a graphical representation of the change in the amount oflight transmitted through the fabric relative to the amount of lightreflected by the fabric as the fabric stretches and recovers duringnormal systole and diastole phases of heart action (FIGS. 2A and 2B),and when a blood vessel underlying the fabric is totally occluded andwhen blood flow is thereafter re-established (FIGS. 2C and 2D);

FIG. 2F is a graphical representation of a signal, periodic in time,representing the changes in the amount of light transmitted through themonitoring fabric relative to the amount of light reflected by themonitoring fabric as represented in FIG. 2E; and

FIG. 3 is a diagram illustrating the temporal relationship betweenpressure measured in an inflatable cuff disposed over an artery of awearer and the voltage appearing of the detector when the method andsystem of the present invention is used to monitor blood pressure of thewearer.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following detailed description similar referencecharacters refer to similar elements in all figures of the drawings.

FIG. 1 is a stylized pictorial representation of a system 10 useful withthe method of the present invention for directly monitoring bloodpressure of a subject by sensing motion due to geometric changes in thebody of the subject associated with the flexing of arterial wallsproduced by blood pressure pulses.

As seen in FIG. 1, the system 10 includes a sleeve 12 having anyconvenient fabric construction (e.g., knitting, weaving) and made fromany suitable textile filament apparel denier yarn. The sleeve 12includes at least a portion, or patch, 14 formed from a monitoringfabric 16. The monitoring fabric 16 has an inner surface 161 andexterior or outer surface 16E. Although the patch 14 is represented asrectangular in shape in FIG. 1, it should be understood that the patch14 may take any convenient or desired shape. If desired, a portion oreven the entirety of the sleeve 12 may be made from the monitoringfabric 16.

The monitoring fabric 16 in accordance with the present inventionexhibits both a light transmission property and a light reflectionproperty when the fabric is illuminated with light having a wavelengthin the extended range from about 400 to about 2200 nanometers. Thisrange is extended in the sense that it encompasses both near infraredlight and visible broad spectrum white light.

As used herein the term “broad spectrum white light” means light havinga wavelength in the range from about four hundred (400) nanometers toabout eight hundred (800) nanometers.

As used herein the term “near infrared light” means light having awavelength in the range from about seven hundred (700) nanometers toabout twenty two hundred (2200) nanometers. The wavelength of 805nanometers or the wavelength of 880 nanometers may be used in systemsoperating in the near infrared spectrum. The wavelength of 805nanometers is preferred.

In accordance with the present invention, the amount of lighttransmitted through the fabric 16 relative to the amount of lightreflected by the fabric 16 is able to change as the fabric stretches.The fabric stretches in response to geometric changes of the body of thesubject due to the flexing of the arteries in response to blood pressurepulses. The term “light balance” may be used herein to refer to theamount of light transmitted through the fabric 16 relative to the amountof light reflected by the fabric 16.

Light reflected by the fabric toward an aperture of acceptance definedwith respect to an axis extending from the surface of the fabric isuseful in producing a signal output from a detector located in theaperture. Alternatively, light transmitted through the fabric is “lost”to a detector placed at the aperture of acceptance.

The monitoring fabric 16 used in the patch 14 can be made from areflective yarns, stretchable yarns any combination of reflective andstretchable yarn or any like material. In one exemplary construction afirst plurality of reflective yarns is combined with a second pluralityof stretchable yarns. The yarns can be combined in any conventionalmanner including woven or non-woven construction.

For woven constructions, yarns can be combined in plain weave, satinweave, twill weave or any other well known constructions. Woven fabricsmay also include weft elastic, warp elastic or bielastic woven fabricsfor varying fabric elasticity.

For non-woven constructions such as knit constructions, yarns can becombined by circular knit, warp knit or any other suitable knitconstruction. In circular knits, typical constructions are single jersey(i.e. different structure in front and back, e.g. 1×1 knit) and doublejersey (i.e. same structure in front and back, e.g. 2×1 knit). Thestitch size and distance determine the openness of the knit fabric. Warpknits may include tricot and raschel constructions where the tightnessis determined by the number of needles/inch or the stitch size.

Any suitable apparel denier and any suitable needle combination orwarp/weft intensity may be used in making the monitoring fabric. Eachreflective yarn may comprise a coating of a specularly reflectivematerial thereon. The coating may also be electrically conductive.Furthermore, the reflective yarn may be elastic. Each stretchable yarnis formed as a combination of an elastic yarn component and a hard yarncomponent.

In the preferred instance, the reflective yarn is that yarn sold byLaird Sauquoit Technologies, Inc. (300 Palm Street, Scranton, Pa.,18505) under the trademark X-STATIC® yarn. X-STATIC® yarn is based upona 70 denier (77 dtex), 34 filament textured nylon available from INVISTANorth America S. àr. l., Wilmington, Del. 19802, as product ID70-XS-34X2 TEX 5Z electroplated with electrically conductive silver.

Alternatively, another method of forming the monitoring fabric 16 is toscreen-print a pattern using an electrically conductive ink afterconstructing the yarns in any conventional woven or non-woven manner.Suitable electrically conductive inks include, but are not limited to,those sold by DuPont Microcircuit Materials, Research Triangle Park,N.C. 27709, as silver ink 5021 or silver ink 5096, and the like.

A screen-printed pattern of conductive inks must also allow the fabricto move. Preferably, the conductive ink does not affect the ability ofthe fabric to stretch and recover. One way to prevent affecting thestretch and recovery properties of fabric is to screen-print a patternof conductive ink(s) in the form of a matrix of dots. Such a dot matrixpattern provides full freedom of movement for the yarns in the fabric,while still exhibiting desired light reflection and transmissionproperties.

The patch 14 of monitoring fabric 16 can alternatively be formed fromelastic and electrically conductive composite yarn comprising a coreyarn made of, for instance, LYCRA® spandex yarn wrapped with insulatedsilver-copper metal wire obtained from ELEKTRO-FEINDRAHT AG,Escholzmatt, Switzerland, using a standard spandex covering process. Thecore yarn may further be covered with any nylon hard yarn or polyesterhard yarn.

Stretchable yarn can be formed in any conventional manner. For example,the stretchable yarn can be formed as a combination of a covered elasticyarn and a hard yarn.

In one preferred embodiment, the covered elastic yarn can be comprisedof a twenty (20) denier (22 dtex) LYCRA® spandex yarn single-coveredwith a ten (10) denier (11 dtex) seven filament nylon yarn. LYCRA®spandex yarn is available from INVISTA North America S. á r. l.,Wilmington, Del. 19805. Alternatively, the elastic yarn component of thepresent invention may comprise elastane yarn or polyester bicomponentyarns, such as those known as ELASTERELL-P™ from INVISTA North AmericaS. á r. l., Wilmington, Del. 19805. The terms spandex and elastane areused interchangeably in the art. An example of a branded spandex yarnsuitable for use with the present invention is LYCRA®.

Synthetic bicomponent multifilament textile yarns may also be used toform the elastic yarn component. One preferred synthetic bicomponentfilament component polymer can be thermoplastic. The syntheticbicomponent filaments can be melt spun or formed in any other mannercommon in the art of filament formation. In the most preferredembodiment, the component polymers can be polyamides or polyesters.

A preferred class of polyamide biocomponent multifilament textile yarnscomprises those nylon biocomponent yarns which are self-crimping, alsocalled “self-texturing”. These bicomponent yarns comprise a component ofnylon 66 polymer or copolyamide having a first relative viscosity and acomponent of nylon 66 polymer or copolyamide having a second relativeviscosity, wherein both components of polymer or copolyamide are inside-by-side relationship as viewed in the cross section of theindividual filament. Self-crimping nylon yarn such as that yarn sold byINVISTA North America S. àr. l, Wilmington, Del, 19805 under thetrademark TACTEL® T-800™ is an especially useful bicomponent elasticyarn.

Some examples of polyester component polymers include polyethyleneterephthalate (PET), polytrimethylene terephthalate (PTT) andpolytetrabutylene terephthalate. In one preferred embodiment, polyesterbicomponent filaments comprise a component of PET polymer and acomponent of PTT polymer in a side-by-side relationship as viewed in thecross section of the individual filament. One exemplary yarn having thisstructure is sold by INVISTA North America S. á r. l., Wilmington, Del.19805 under the trademark T-400™ Next Generation Fiber.

The hard component could be made from any inelastic synthetic polymerfiber(s) or from natural textile fibers, such as wool, cotton, ramie,linen, rayon, silk, and the like. The synthetic polymer fibers may becontinuous filament or staple yarns selected from the multifilament flatyarns, partially oriented yarns, textured yarns, bicomponent yarnsselected from nylon, polyester or filament yarn blends. The hardcomponent is preferably 260 denier (286 dtex) 68 filament nylon yarn.

Nylon yarns may comprise synthetic polyamide component polymers, such asnylon 6, nylon 66, nylon 46, nylon 7, nylon 9, nylon 10, nylon 11, nylon610, nylon 612, nylon 12 and mixtures and copolyamides thereof. In thecase of copolyamides, especially preferred are those including nylon 66with up to 40 mole percent of a polyadipamide wherein the aliphaticdiamine component is selected from the group of diamines available fromE.I. Du Pont de Nemours and Company of Wilmington, Del. under therespective trademarks DYTEK A® and DYTEK EP®.

Further in accordance with the present invention, the hard yarn portionmay comprise polyesters such as, for example, polyethylene terephthalate(PET), polytrimethylene terephthalate (PTT), polybutylene terephthalateand copolyesters thereof.

The monitoring fabric 16 may also be formed from composite yarns inwhich the reflective and stretchable components are combined in the sameyarn. Such a composite yarn would include a covering yarn having aspectrally reflective outer surface that is wrapped about an elasticyarn component in one or more layers.

The remainder of the structure of the sleeve 12, if not also formed ofthe monitoring fabric, may exhibit any convenient textile construction(e.g. knitting or weaving as described above), and may be made from anysuitable textile filament apparel denier yarn.

In one embodiment, the monitoring fabric 16 used in the patch 14 isattached to the sleeve 12. The patch 14 could be sewn, glued, taped,buttoned, interwoven or attached to the sleeve by any conventionalmeans. In another embodiment, the sleeve 12 is completely constructed ofthe monitoring fabric 16.

The present invention is directed to monitoring the light balance of amonitoring fabric 16 as it stretches and recovers. For this purpose, thesystem 10 includes a suitable source 18 of radiation operable in thewavelength range from about 400 nanometers to about 2200 nanometers, andparticularly in the ranges from about 400 to about 800 nanometers andfrom about 700 to about 2200 nanometers. An associated detector 22 isresponsive to incident radiation in the given wavelength range andsubranges for producing signals in response thereto.

In the case of operation with near infrared light, the radiation source18 can be a compound semiconductor-based (e.g., gallium arsenide orgallium aluminum arsenide) photo-emitting diode operating in theinfrared range (at a wavelength of, for example, 805 nanometers or 880nanometers). The detector 22 can be any device that can detectradiation, for instance, a photodiode coupled to appropriatelyconfigured output amplification stages. Any well known semiconductorscan be used for forming the photodiode including silicon or germanium. Acommercially available radiation source and detector package suitablefor use in the system of the present invention is that available fromFourier Systems Ltd. (9635 Huntcliff Trace, Atlanta, Ga., 30350) asmodel DT155 (0-5 volt output).

For broad spectrum white light (400 to 800 nanometers) operation, thesource 18 can be a compound semiconductor-based “white LED” (e.g., alight emitting diode employing an indium gallium nitride based devicewith suitable phosphors to provide broad spectrum white light emission).An LED source of broad spectrum white light is available from Lumitex®Inc., 8443 Dow Circle, Strongsville, Ohio 44136, USA; Part No. 003387.Such an LED provides radiation in the wavelength range of 430 to 700nanometers. The detector 22 is a preferably a silicon phototransistorcoupled to appropriately configured output amplification stages.

The radiation source 18 and the detector 22 are attached to monitoringfabric 16 in predetermined relative positions. The positions weredetermined such that the reception of incident radiation by the detector22 is directly affected by a change in the amount of light transmittedthrough the monitoring fabric 16 relative to the amount of lightreflected by the monitoring fabric 16 when the fabric stretches andrecovers. In the preferred case, the radiation source 18 and detector 22are embedded, or fixed firmly, into the textile structure of themonitoring fabric 16. The radiation source 18 and detector 22 can befixed using any well known attachment method, including, but not limitedto, clamping, gluing, sewing, taping, or hook and loop fasteners(Velcro). Optionally, it may be desirable in some operationalconfigurations of the invention to dispose both the source and thedetector remotely from and not in direct contact with the fabric 16. Insuch a remote arrangement, the radiation source 18 and detector 22 couldbe located in any arrangement that permits the detector 22 to detectchanges in the transmission and reflection of radiation duringstretching and recovery.

The source 18 may be arranged in such a way as to maintain its relativeposition to the detector 22. For instance, the source 18 and thedetector 22 may be rigidly connected together on one side of the fabric16 to maintain a spatial relationship. Alternatively, the position ofthe source 18 relative to the detector 22 can be maintained on oppositesides of the monitoring fabric 16 for monitoring light transmission. Insuch an embodiment, the radiation source is connected to the radiationdetector using a “clothes-pin” or alligator style clamp. Other wellknown means of maintaining spatial relationship between the source andthe detector are contemplated.

The apparatus 10 represented in FIG. 1 further includes an inflatablecuff 28 substantially similar to a standard cuff known in the bloodpressure measurement art. The cuff 28 is typically a woven nylon sleevewith a seamless inner bladder 30 of latex rubber. The inner bladder 30communicates with an external air pump 32, valve 34 and a pressuremeasurement device 36 for measuring the internal pressure of the bladder30. The pressure measurement device 36 communicates electrically withthe processor 26 over an electrical connection 38. The valve 34 may beconfigured for automatic operation by the signal processor 26 over acontrol line 40.

The pump 32 may take the form of any conventional pump including amanually operated device, an automated piston or diaphragm air pump. Thepressure measurement device 36 may be implemented using a manometer ofknown design. Such a manometer is constructed using known barometricmeasurement techniques that include a column of mercury, a Bourdon gaugemovement, and a measurement device having a high reproducibility ofmeasurement in the expected range of operation, for example, from aboutten (10) millimeters of mercury to about five hundred (500) millimetersof mercury (mm Hg). A suitable pressure measurement device 36 could be adirect capacitance measurement device provided with a fully electronicoutput suitable for use with traditional digital signal processors. Apressure transducer operable to measure the pressure within the cuff atpredetermined sampling time intervals is also suitable for use as thepressure measurement device. The miniature silicon pressure sensorpackage available from Advanced Custom Sensors, Inc. Irvine, Calif.,92618 as Model 7277 (zero to five volt output; zero to seven pounds persquare inch gage rating) is such a device, though any similar means formeasuring pressure is contemplated.

The principles of operation by which the motion of a subject's body dueto geometric changes generated by blood pressure pulses may be monitoredin accordance with the system of the present invention may be moreclearly understood with reference to FIGS. 2A through 2F. In thediscussion that follows both the source 18 and the detector 22 aremounted adjacent to the same surface 16E of the monitoring fabric 16 soas to operate in a “reflection mode”. Alternatively, it is contemplatedwithin the scope of the invention to operate in a “transmission mode”.In the alternative transmission mode of operation, the source 18 and thedetector 22 are mounted to opposite sides of the monitoring fabric 16.

The reaction of the fabric 16 during normal diastole and systole phasesof heart action are depicted in FIGS. 2A and 2B.

As represented in FIG. 2A in a normal diastole phase the yarns 16Yforming the monitoring fabric 16 lie within a relatively close distanceof each other to define a pattern of relatively narrow gaps 16G. Agenerally circular spot indicated by the reference character 17represents the area of the monitoring fabric 16 illuminated by thesource 18. Of the photons emitted from the radiation source 18 towardthe surface 16E of the fabric 16 some photons are absorbed (e.g.,represented by a ray 18C) by the yarns 16Y of the fabric while otherphotons (e.g., the rays 18A and 18B) pass through gaps 16G therein.These photons are lost to the detector 22. The major portion of thelight (e.g., represented by the rays 18D through 18G) is reflected fromthe surface 16E of the monitoring fabric 16 toward the detector 22. Thismajor portion of the light is detected by detector 22, which in turnproduces a corresponding output signal.

During a normal systole phase of a heart beat, the size of the gaps 16Gformed in the monitoring fabric 16 increases in response to motioninduced by the flexure of the underlying blood vessels. This increase insize of the gaps 16G (FIG. 2B) increases the likelihood that a photonwill pass through the fabric 16, and decreases the likelihood that aphoton will reflect toward the detector 22. The total number of photonslost to the detector 22 by transmission through or absorption by thefabric (e.g., represented by the rays 18A, 18B and 18C) increases. Thesignal output from the detector 22 concomitantly decreases. Although thenumber of photons lost to the detector 22 by absorption (e.g.,represented by the ray 18C) does not necessarily change, the likelihoodthat a photon will strike a yarn 16F and be reflected or absorbeddecreases since the spot size 17 remains constant in the area while thegap 16G size increases.

As the systolic phase of the pulse beat gives way to the diastolicphase, the fabric 16 undergoes the elastic recovery. The gaps 16G returnto their original size (FIG. 2A). The major portion of the light isagain reflected toward the detector 22, increasing the output signaltherefrom.

The left hand portion of FIG. 2E illustrates the waveform of the signalgenerated at the detector 22 as the fabric undergoes its stretch cyclefrom the initial diastolic phase (represented by the reference character“I”) through a systolic phase (represented by the reference character“II”) and back to the diastolic phase (“I”). This portion of FIG. 2Egraphically illustrates that during the course of a stretch cycle thelight balance (reference character “LB” in FIG. 2E) of the fabricchanges.

As shown in FIG. 2E, at diastolic phase (“I”) of FIG. 2A, the reflectedlight represented by the bottom portion below the “LB” is greater thanthe transmitted light represented by the upper portion above the “LB”.In contrast, FIG. 2E shows that at systolic phase (“II”) of FIG. 2B, thereflected light represented by the bottom portion below the “LB” is lessthan the transmitted light represented by the upper portion above the“LB”.

Comparison between the diastolic and systolic phases indicates that theamount of light transmitted through the monitoring fabric 16 relative tothe amount of light reflected by the monitoring fabric 16 changes in aperiodic fashion over time as the fabric stretches. Light lost to thedetector 22 by absorption may be considered as contributing to the“transmitted light” section of the graph of FIG. 2E. This periodicvariation in light balance is represented in the left-hand portion ofFIG. 2F as a time-varying signal synchronized with the elongation andrecovery stages of fabric stretch (e.g., diastolic phase represented bycharacter “I” and systolic phase represented by character “II”).

The situation that arises when a cuff is fully pressurized is depictedin FIG. 2C. In this case the flow of blood is completely interrupted andno flexure of the underlying artery occurs. The fabric 16 reverts to afully unstretched condition with the gap spacing 16G between yarns 16Ybeing at its minimum. Substantially all of the photons falling on theilluminated spot 17 are reflected toward the detector 22 resulting inthe light balance depicted at reference character “III” in the righthand portion of FIG. 2E. At reference “III”, the reflected lightrepresented by the bottom portion below the “LB” is significantlygreater than the transmitted light represented by the upper portionabove the “LB”.

FIG. 2D depicts the situation when blood flow through the previouslyoccluded artery is re-established. The sudden rush of blood through theartery elongates the fabric 16 and extends the gaps 16G to a widerextent than in the systolic phase (shown in FIG. 2B). The amount oflight transmitted through the fabric 16 increases commensurately whilethe amount of light reflected significantly decreases. This result isillustrated in the change in light balance depicted at referencecharacter “IV” in the right hand portion of FIG. 2E. In FIG. 2E, atreference character “IV”, the reflected light represented by the bottomportion below the “LB” is less than the transmitted light represented bythe upper portion above the “LB”. The right hand portion of FIG. 2Fillustrates the rapid decrease in signal output from the detector 22caused by the sudden rush of blood through the previously occludedartery (e.g., the change in detector output voltage from phase “III” tophase “IV” shown in FIG. 2F)

As the rush of blood diminishes the light balance and the signal outputfrom the detector 22 would revert toward the ranges exhibited duringnormal diastolic and systolic operation.

Operation

In an exemplary mode of operation the apparatus of the present inventionwould be applied to the arm of a person generally in the positionindicated by the dotted outline in FIG. 1, with the fabric patch 14 ofsleeve 12 over the brachial artery. The inflatable cuff 28 is appliedover the sleeve 12.

FIG. 3 illustrates the temporal relationship between pressure measuredin the cuff 28 and a voltage resulting from the radiation detected bythe detector 22 in reflection mode. In a reflection mode of operation,the source 18 and the detector 22 are arranged adjacent to side 16E ofthe fabric 16. As discussed in connection with FIGS. 2A through 2F, anyfabric 16 elongation is accompanied by a decreasing signal (voltage)from the detector 22, corresponding to increased light transmissionthrough the fabric 16.

Considering again FIG. 3, the pressure in the cuff 28 is at atmosphericpressure P₀ during the time interval between t₀ and t₁. Also, duringthis same time interval the signal at detector 22 is a mean value V_(N)varying between the limits of (V_(N)+Δ and V_(N)−Δ)

This mean voltage V_(N) results from the subtle flexing of the arterialwalls and subsequent subtle movement of tissue underlying the fabric 16.This subtle movement is due to the arterial pressure fluctuations fromthe systolic to the diastolic pressure extremes. Because blood flow islaminar and silent during this interval, the net average force on thearterial walls is constant. During the arbitrary time interval t₀ to t₁,the voltage reading from detector 22 is sampled; and a voltage V_(N)(+/−Δ) is stored in memory of the processor 26.

Next, at time t₁, under control of the processor 26, the valve 34 isclosed and the pump 32 activated. Accordingly, during the time intervalfrom t₁ to t₂, the pressure in the cuff 28 rises to a pressure P_(H), apressure value chosen to be above the highest expected systolic pressure(e.g. 300 to 330 mm Hg). This pressure value is also stored in memory ofthe processor 26. During this time interval, the brachial arterial bloodflow becomes totally interrupted. Effectively, there is no arterial wallflexing or underlying body tissue movement because blood flow isoccluded. As explained in connection with FIG. 2C, the voltage outputfrom the detector 22 rises to V_(H).

During the time interval from t₁ to t₂, the cuff pressure and detectorvoltage are sampled and stored at a fixed sampling frequency (chosen ina frequency range from ten (10) Hertz to ten thousand (10,000) Hertz).The voltage reading from detector 22 when the pressure in cuff 28reaches its maximum value P_(H) (at time t₂ in FIG. 3) is stored inprocessor 26.

At t₂ the pressure in the cuff 28 is released by opening valve 34 undercontrol of the processor 26. During the time interval t₂ to t₃ thevoltage output from detector 22 and the pressure in the cuff 28 aresampled and stored at the sampling frequency. During the interval t₂ tot₃ the rate of pressure change within the cuff 28 is about two (2) mm Hgto about six (6) mm Hg per second.

As the cuff pressure is reduced, at some time within the interval t₂ tot₃ the pressure in the cuff 28 is equal to the systolic (highest)arterial blood pressure and the artery snaps open with a sudden flow ofblood. In turn, as explained in connection with FIG. 2D, the voltage atthe detector 22 experiences a maximum in rate of change and falls to amaximum deviation from V_(N), to a voltage value V_(L). As shown in FIG.3, this voltage V_(L), detected at t₃, corresponds to the sudden rush ofarterial blood filing the formerly occluded artery with a sharp flexingof the artery walls. This sharp flexing of the walls in turn stimulatesmovement of the tissue overlying the artery and underlying the fabric16. This movement corresponds to a maximum amplitude for the motion ofthe tissue underlying the fabric 16. At t₃, corresponding to voltageV_(L), this maximum amplitude in tissue movement forces maximum fabric16 flexing, sudden elongation, signaling that a peak or systolicpressure P_(S) is present in the brachial artery.

During the interval of time t₃ to t₄ the pressure in the cuff 28 isfurther decreasing. The voltage and pressure values are sampled at thepredetermined frequency. The voltage output at the detector 22 continuesto rise during this time interval t₃ to t₄.

Following the occurrence of the minimum voltage V_(L), the outputvoltage of the detector again returns to within the predetermined rangeabout the baseline value (within the limits (V_(N)+Δ and V_(N)−Δ) andthe pressure in the cuff 20 passes through a point of equivalence withthe diastolic (lowest) pressure in the brachial artery, uniquelydefining t₄. Any further drop in pressure in the cuff 28, to atmosphericpressure, produces no further rise in voltage at detector 22 during thetime interval t₄ to t_(t).

The system and method of the present invention may alternatively beoperated in a transmission mode where the source 18 is disposed on side16E of the fabric 16 while the detector 22 is disposed on the oppositeside 16I. The dot-dashed outline of detector 22, shown in FIGS. 2Athrough 2D represents the location of detector 22 in transmission mode.The light balance between reflection and transmission will be the sameas the situation discussed in connection with the reflection mode.However, the detector output voltage shown in FIG. 2F is reversed sincemore fabric stretch results in higher light transmission providing acommensurately higher detector voltage. The detector voltage waveformshown in FIG. 3 is therefore reversed or inverted as well. In contrastto the reflection mode shown in FIG. 3, the transmission mode measures adecrease in voltage between the interval of time t₁ to t₂ and anincrease between the interval of time t₂ to t₃. Finally, in contrast toFIG. 3 the detector voltage decreases between t₃ and t₄ to return tovoltage V_(N).

The measurement of pressures P_(S) and P_(D) is, in principle,determined by means of signal processing techniques known in the art.For example, one commercially available pressure transducer 36, Model7277 from Advanced Custom Sensors, Inc. Irvine, Calif., 92618, is aminiature silicon pressure sensor package, with 0 to 5 volt output, and0 to 7 pound per square inch gage rating. This pressure transducer 36will communicate directly with signal processor 26 such as a Z8®microcontroller, Model Z86C08 from ZILOG, Inc., Campbell, Calif.,95008-6600 with electronically programmable memory and associatedcircuitry.

Those skilled in the art, having the benefit of the teachings of thepresent invention as hereinabove set forth, may effect modificationsthereto. Such modifications are to be construed as lying within thescope of the present invention, as defined by the appended claims.

1. A system for monitoring blood pressure comprising: an inflatable armcuff that is selectably inflatable to differing air pressures; a patchdisposed in or on said arm cuff comprising fabric that has both a lighttransmission property and a light reflection property that change whenthe fabric stretches; a radiation source mounted to the fabric, whereinsaid radiation source directs radiation onto the fabric; and a radiationdetector mounted to the fabric.
 2. The system of claim 1, wherein theradiation source emits radiation with a wavelength in the range of fromabout 400 to about 2200 nanometers; and the detector responds toincident radiation with a wavelength in the range of from about 400 toabout 2200 nanometers.
 3. The system of claim 1, wherein the source anddetector are attached to the fabric in relative positions such that thereception of incident radiation by the detector is directly affected bya change in the amount of light transmitted though the fabric relativeto the amount of light reflected by the fabric as the fabric stretchesin response to motion in the body of a wearer due to changes in the flowof blood through an artery disposed beneath the patch occurring inconsonance with variations in the air pressure within the inflatablecuff.
 4. The system of claim 1, wherein the fabric is integral with thecuff.
 5. The system of claim 1 wherein the fabric comprises a firstplurality of reflective yarns woven or knit with a second plurality ofstretchable yarns.
 6. The system of claim 1 wherein the fabric has afirst and a second side; and wherein the source and the detector aremounted on a same side of the fabric.
 7. The system of claim 1 whereinthe fabric has a first and a second side; and wherein the source and thedetector are mounted on opposing sides of the fabric.
 8. A system formonitoring blood pressure of a wearer, the system being usable with aninflatable arm cuff, the arm cuff being sized for receipt over a portionof the body of a wearer, the cuff being selectably inflatable todiffering air pressures to modify the flow of blood through an arterylocated in the portion of the body of a wearer over which the cuff isdisposed, the blood pressure monitoring system comprising: a patch atleast a portion of which is formed from a fabric which has both a lighttransmission property and a light reflection property when the fabric isilluminated with light and in which the amount of light transmittedthrough the fabric relative to the amount of light reflected by thefabric is able to change when the fabric stretches in response togeometric changes in the body of a wearer due to changes in the flow ofblood through an artery disposed beneath the patch; a source ofradiation with a wavelength in the range from about 400 to about 2200nanometers; a detector that produces a signal in response to incidentradiation with a wavelength in the range from about 400 to about 2200nanometers; the source and detector being attached to the fabric inrelative positions such that the reception of incident radiation by thedetector is directly affected by a change in the amount of lighttransmitted through the fabric relative to the amount of light reflectedby the fabric as the fabric stretches in response to motion in the bodyof a wearer due to changes in the flow of blood through an arterydisposed beneath the patch occurring in consonance with variations inthe air pressure within the inflatable cuff; and a pressure recorderresponsive to the signal output from the detector for recording thepressure of the cuff when the output of the detector is at a minimumvalue, and for recording the pressure in the cuff following the minimumvalue when the signal from the detector again lies within apredetermined range of a baseline signal value.
 9. The monitoring systemof claim 8, wherein the fabric has a first and a second side; andwherein the source and the detector are mounted on opposing sides of thefabric.
 10. The monitoring system of claim 8 wherein the fabric has afirst side and a second side; and wherein the source and the detectorare mounted on the same side of the fabric.